A field trial was conducted in Dorset to determine whether calves could be protected against babesiosis by the prior inoculation of irradiated blood infected with Babesia divergens. The trial involved 99 Friesian heifers. Forty were inoculated with infected blood from a donor calf after the blood had been irradiated to a dose of 28 kilorads, 31 were immune animals which had grazed on the infected site the previous year and 28 were susceptible animals. After exposure on a hillside where there was a high risk of contracting the disease, 21 of 28 susceptible animals became clinically affected by redwater and required treatment whereas no animals in the other two groups were affected. It was shown that there was a 100 per cent risk of disease at the site and serological and haematological evidence corroborated the clinical results in demonstrating total protection of the vaccinated animals. Vaccination against redwater appears preferable to therapy, since vaccination before exposure on an infected pasture can replace frequent monitoring of the animals and subsequent therapy late in the course of the disease.
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